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<a name="SYSTEM_005fCLOCK"></a>
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<a name="SYSTEM_005fCLOCK-_002d_002d_002d-Time-function"></a>
<h3 class="section">8.264 <code>SYSTEM_CLOCK</code> &mdash; Time function</h3>
<a name="index-SYSTEM_005fCLOCK"></a>
<a name="index-time_002c-clock-ticks-2"></a>
<a name="index-clock-ticks-2"></a>

<dl compact="compact">
<dt><em>Description</em>:</dt>
<dd><p>Determines the <var>COUNT</var> of a processor clock since an unspecified
time in the past modulo <var>COUNT_MAX</var>, <var>COUNT_RATE</var> determines
the number of clock ticks per second.  If the platform supports a
monotonic clock, that clock is used and can, depending on the platform
clock implementation, provide up to nanosecond resolution.  If a
monotonic clock is not available, the implementation falls back to a
realtime clock.
</p>
<p><var>COUNT_RATE</var> is system dependent and can vary depending on the kind of
the arguments. For <var>kind=4</var> arguments (and smaller integer kinds),
<var>COUNT</var> represents milliseconds, while for <var>kind=8</var> arguments (and
larger integer kinds), <var>COUNT</var> typically represents micro- or
nanoseconds depending on resolution of the underlying platform clock.
<var>COUNT_MAX</var> usually equals <code>HUGE(COUNT_MAX)</code>. Note that the
millisecond resolution of the <var>kind=4</var> version implies that the
<var>COUNT</var> will wrap around in roughly 25 days. In order to avoid issues
with the wrap around and for more precise timing, please use the
<var>kind=8</var> version.
</p>
<p>If there is no clock, or querying the clock fails, <var>COUNT</var> is set
to <code>-HUGE(COUNT)</code>, and <var>COUNT_RATE</var> and <var>COUNT_MAX</var> are
set to zero.
</p>
<p>When running on a platform using the GNU C library (glibc) version
2.16 or older, or a derivative thereof, the high resolution monotonic
clock is available only when linking with the <var>rt</var> library.  This
can be done explicitly by adding the <code>-lrt</code> flag when linking the
application, but is also done implicitly when using OpenMP.
</p>
<p>On the Windows platform, the version with <var>kind=4</var> arguments uses
the <code>GetTickCount</code> function, whereas the <var>kind=8</var> version
uses <code>QueryPerformanceCounter</code> and
<code>QueryPerformanceCounterFrequency</code>. For more information, and
potential caveats, please see the platform documentation.
</p>
</dd>
<dt><em>Standard</em>:</dt>
<dd><p>Fortran 90 and later
</p>
</dd>
<dt><em>Class</em>:</dt>
<dd><p>Subroutine
</p>
</dd>
<dt><em>Syntax</em>:</dt>
<dd><p><code>CALL SYSTEM_CLOCK([COUNT, COUNT_RATE, COUNT_MAX])</code>
</p>
</dd>
<dt><em>Arguments</em>:</dt>
<dd><table>
<tr><td width="20%"><var>COUNT</var></td><td width="65%">(Optional) shall be a scalar of type 
<code>INTEGER</code> with <code>INTENT(OUT)</code>.</td></tr>
<tr><td width="20%"><var>COUNT_RATE</var></td><td width="65%">(Optional) shall be a scalar of type 
<code>INTEGER</code> or <code>REAL</code>, with <code>INTENT(OUT)</code>.</td></tr>
<tr><td width="20%"><var>COUNT_MAX</var></td><td width="65%">(Optional) shall be a scalar of type 
<code>INTEGER</code> with <code>INTENT(OUT)</code>.</td></tr>
</table>

</dd>
<dt><em>Example</em>:</dt>
<dd><div class="smallexample">
<pre class="smallexample">PROGRAM test_system_clock
  INTEGER :: count, count_rate, count_max
  CALL SYSTEM_CLOCK(count, count_rate, count_max)
  WRITE(*,*) count, count_rate, count_max
END PROGRAM
</pre></div>

</dd>
<dt><em>See also</em>:</dt>
<dd><p><a href="DATE_005fAND_005fTIME.html#DATE_005fAND_005fTIME">DATE_AND_TIME</a>, <br>
<a href="CPU_005fTIME.html#CPU_005fTIME">CPU_TIME</a>
</p></dd>
</dl>



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